Conventionally, aromatic diisocyanates are prepared by phosgenation of the corresponding diamino compounds. The use of phosgene results in the generation of hydrochloric acid as a side product, which is the cause of severe corrosion. Considering the environmental hazard and corrosion problems in the phosgenation route, it is important to develop a route without using phosgene.
MDI is an important starting material for the production of polyurethanes that are useful as elastomers, artificial leather, coatings, spandex fibers etc. Diphenyl methane dicarbamates are useful precursors for the preparation of diphenylmethane diisocyanates (MDI).
Various methods for the preparation of alkyl and aromatic carbamates by carbonylation of corresponding amino compounds with carbon monoxide and oxygen using Group VIII metal catalysts have been reported (U.S. Pat. Nos. 5,502,241; 5,194,660; 4,694,097 and 4,242,520; EP-A 83096).
Asahi Chemicals have proposed a process for the preparation of 4,4′-MDU (U.S. Pat. Nos. 4,552,974; 4,230,877; 4,319,018; GB-A 2054584; JP-A 12357/81) by condensation of ethyl N-phenyl carbamate with formaldehyde in the presence of acids, such as mineral acids or organic sulfonic acids. This requires relatively severe reaction conditions.
Recently, Valli and Alper have published a work on oxidative carbonylation of various aliphatic diamines to aliphatic diurethanes using Pd-clay/NaI catalyst system (Organomett. 14, 81 (1995)).
Oxidative carbonylation of aniline to the corresponding aromatic urethane has already been disclosed in Catalysis Letters (1994), 25, p. 361–364.
However, none of the prior art documents report catalytic conversion of polymeric amino compounds to their corresponding carbamate derivatives by a clean and environmentally benign route.